Limited slip differentials provide various degrees of torque transfer through the differential and many examples of limited slip differential are contained in prior art. Limited slip differentials are used in many power transmission devices including axles, transfer case units, all wheel drive units and transmissions. The complexity, size and weight of currently available torque biasing differentials limit their use and placement within many of these power transmission devices.
Torque sensing limited slip differentials require some degree of torque difference across the differential before the torque bias ratio of the differential is increased or decreased. Conversely, speed sensing limited slip differentials require some degree of speed difference across the differential before the torque bias ratio is increased or decreased. These requirements result in less than optimum compatibility with active traction enhancement and electronic stability control programming.
One limited slip differential described in prior art utilizes a clutch pack to transfer torque through the differential. In some applications of this design, the clamping action of the clutch pack increases as the input torque through the differential increases. In another example, as the speed difference through the differential increases, the differential case will spin and force a pressure ring out against the clutch plates thereby squeezing them together, increasing the torque transfer through the differential. The overall effect is to limit wheel spin when one of the tires is on a surface with lower available traction by transferring torque to the tire with greater available traction. One of the disadvantages of plate clutch type limited slip differentials is that the effectiveness of the device declines as the clutch packs wear. In addition, limited slip differentials using clutch packs are subject to chatter which results from a stick-slip condition occurring between the clutch plates.
Applications of torque biasing differentials include helical gear type differentials. A torque bias differential can increase the applied torque through the differential almost instantaneously. As the input torque increases the torque bias differential supplies a torque bias based on a fixed ratio that is a function of the design of the torque bias differential. This ratio cannot be altered without physically modifying the differential thereby limiting the torque bias differential's effectiveness as an active component in electronic stability systems and ABS systems.
Other torque biasing devices employing magnetorheological fluids require a pump to move the magnetorheological fluid through a tube or other external structure. Pumping of the magnetorheological fluid results in greater degradation of the magnetorheological fluid and increased wear within the system. In addition, the requirement of pumping the magnetorheological fluid increases the size and complexity of a torque biasing device as well as the system response time required achieve the desired torque biasing.
It is therefore desirable to provide an active torque biasing differential that overcomes the limitations, challenges, and obstacles described above.